Stephen J. Picken

Responsive biomimetic networks from polyisocyanopeptide hydrogels

Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells. The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen. As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum. Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model, we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications, in particular in the biomedical field.

Homologous series of liquid-crystalline metal free and copper octa-n-alkoxyphthalocyanines

Abstract Two series of discotic liquid crystals, one based on metal-free and the other on copper containing phthalocyanine (Pc) as the central core and eight peripherally n-alkoxy groups (C6–C12) as substituents have been synthesized and their mesomorphic properties and structural parameters have been determined. These compounds represent a series totally devoid of polymorphism, the mesophase in all cases being Dho. In the solid state an orthorhombic two dimensional lattice with four molecules per unit cell and a presumably tilted stacking is present, while in the mesophase the planes of the molecules are on average perpendicular to the direction of the stacks. The aggregation behaviour of the C8, C10 and C12 homologues was studied as a function of solvent polarity and of concentration. The electrical properties of the copper complexes of the n-dodecoxy and n-octoxy derivatives have been determined by means of complex impedance spectroscopy and compared with those of the non-mesogenic CuPc.

A molecular dynamics study of the nematic phase of 4-n-pentyl-4'-cyanobiphenyl

Abstract Preliminary molecular dynamics simulations of the nematic phase of 4-n-pentyl-4′-cyanobiphenyl are described. The simulations include all molecular degrees of freedom. The influence of the molecular dipole moment is investigated by comparing simulations with and without a charge distribution on the molecules. Inclusion of the charge distribution is found to lead to a slight broadening of the orientational distribution function, in qualitative agreement with Raman measurements of the orientational order parameters.

Peripherally Octasubstituted Phthalocyanines with Branched Alkoxy Chains

Abstract Phthalocyanines (H2Pc) octasubstituted with branched alkoxy chains were synthesized: 2,3,9,10,16,17,23,24- octakis(3,7-dimethyloctoxy)phthalocyanine (1a) and 2,3,9,10,16,17,23,24-octakis(3,7,11-trimethyldodecoxy)phthalocyanine (1b). Their mesomorphic properties were studied by DSC, polarization microscopy and X-ray diffraction. Compound 1a, which was initially crystalline at room temperature, maintains its mesomorphic structure on cooling back to room temperature. Compound 1b is mesomorphic at room temperature. It shows a D ho→D hd transition at 34[ddot]C and a transition of unknown nature at 173[ddot]C. Both compounds display a D→I transition.

Enhanced hardening of soft self-assembled copolymer gels under homogeneous magnetic fields

The rheological response of highly swollen physical gels obtained by self-assembling of triblock copolymers containing low remanence ferromagnetic particles was investigated in the presence of external homogeneous magnetic fields. Three different types of sample geometries with distinctive magnetic particle orderings were investigated: isotropic (no magnetic field present during synthesis), parallel to the plane of the gel film and perpendicular to the plane of the gel film. Both the storage and loss moduli exhibit a strong increase with magnetic field strength for all geometries. Dependence of the rheological response on particle volume fraction was also investigated. The strength of such rheological hardening, as well as its saturation behaviour, depend strongly on the relative orientation between particle strings, shear and external field. In some cases a very strong relative increase of storage modulus, up to 6000% was obtained. Further transient rheological studies suggest that strong rearrangement of the particle network is largely responsible for the enormous increase in elastic modulus. Parallel to that, a maximum in the loss factor was observed as a function of particle volume fraction and field strength and it was interpreted in terms of a competition between an increase in string (clusters) hardening and a decrease in their ability to deform and flow. These results suggest that magnetorheological gels are an intermediate system between magnetorheological elastomers (MREs) and magnetorheological fluids (MRFs) with directional dependent rheological response and partial rearrangement of the particle network.

Simultaneous electrochemical determination of nitrate and nitrite in aqueous solution using Ag-doped zeolite-expanded graphite-epoxy electrode

In this work a new electrochemical sensor based on an Ag-doped zeolite-expanded graphite-epoxy composite electrode (AgZEGE) was evaluated as a novel alternative for the simultaneous quantitative determination of nitrate and nitrite in aqueous solutions. Cyclic voltammetry was used to characterize the electrochemical behavior of the electrode in the presence of individual or mixtures of nitrate and nitrite anions in 0.1M Na(2)SO(4) supporting electrolyte. Linear dependences of current versus nitrate and nitrite concentrations were obtained for the concentration ranges of 1-10mM for nitrate and 0.1-1mM for nitrite using cyclic voltammetry (CV), chronoamperometry (CA), and multiple-pulsed amperometry (MPA) procedures. The comparative assessment of the electrochemical behavior of the individual anions and mixtures of anions on this modified electrode allowed determining the working conditions for the simultaneous detection of the nitrite and nitrate anions. Applying MPA allowed enhancement of the sensitivity for direct and indirect nitrate detection and also for nitrite detection. The proposed sensor was applied in tap water samples spiked with known nitrate and nitrite concentrations and the results were in agreement with those obtained by a comparative spectrophotometric method. This work demonstrates that using multiple-pulse amperometry with the Ag-doped zeolite-expanded graphite-epoxy composite electrode provides a real opportunity for the simultaneous detection of nitrite and nitrate in aqueous solutions.

Oligomeric rod–disc nematic liquid crystals

Six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers.

Electric field poling of acceptor–donor molecules

The Maier–Saupe theory for liquid crystals is extended to describe poling by an electric field of side-chain polymers with π-conjugated acceptor–donor molecules as pendant groups. Applications of the theory are in the field of nonlinear-optical materials. By using a quasi-one-dimensional description of the acceptor–donor molecules, our model is capable of going beyond the linear approximation and is intended for the strong-field regime. The theory takes both the dipole moment and the anisotropic polarizability of the side chains into account. A field-induced phase transition to the nematic phase is predicted for isotropic materials. This leads to enhanced polar order and thus to a larger nonlinear susceptibility than is predicted by the simpler isotropic model. The difference between initially liquid-crystalline and initially isotropic materials becomes smaller if the field-induced axial order is taken into account.

Uniaxial to biaxial nematic phase transition in a bent-core thermotropic liquid crystal by polarising microscopy

Polarising optical microscopy (POM) shows evidence for a reversible uniaxial nematic (NU) to biaxial nematic (NB) phase transition in a quiescent melt of bent-core mesogens (BCM). A second superimposed Schlieren texture, attributed to a second director field, grows into the NU phase on lowering the temperature and disappears again on heating, indicating a fully reversible NB to NU phase transition. This POM result together with recent X-ray data provides a new signature for the NB phase in this class of BCMs, and offers a new method for assessment of new biaxial nematic phases.

Mobility and solubility of antioxidants and oxygen in glassy polymers. III. Influence of deformation and orientation on oxygen permeability

The mobility of small molecules in a glassy polymer is largely determined by the amount of free volume present in the material. The amount of free volume can be altered by changing the physical state of the polymer. Plastic deformation under compression reduces this amount, whereas the application of a tensile stress increases it. Furthermore, orientation of a polymer introduces an anisotropy in the free volume. The change in free volume was monitored by oxygen permeation experiments. A clear correlation was found between the draw ratio, plastic deformation and stress on the one hand and oxygen permeability on the other. Since the mobility of oxygen is an important parameter for the stabilisation of a polymer against oxidation, the physical state of the polymer can have a significant influence on the service life of the product.